Flow measurement has been an essential requirement in many commercial and industrial
applications; one of the urgent needs is the low-power flow-sensing node for the Internet of
Things (IoT) in the smart Energy-efficient Building (EeB). With the mature CMOS fabrication
techniques, the low-cost MEMS flow devices combined with the highly integrated circuits (ICs)
and micro mechanical components are able to be carried out. In particular, the target of this
thesis, thermal flow sensors do not require any moving parts that make them perhaps the easiest
flow devices to be implemented in the CMOS process due to the structural and electronic
simplicity.
This thesis mainly focuses on the systematic design and fabrication of the low-cost
micromachined calorimetric flow sensors by using t...[
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Flow measurement has been an essential requirement in many commercial and industrial
applications; one of the urgent needs is the low-power flow-sensing node for the Internet of
Things (IoT) in the smart Energy-efficient Building (EeB). With the mature CMOS fabrication
techniques, the low-cost MEMS flow devices combined with the highly integrated circuits (ICs)
and micro mechanical components are able to be carried out. In particular, the target of this
thesis, thermal flow sensors do not require any moving parts that make them perhaps the easiest
flow devices to be implemented in the CMOS process due to the structural and electronic
simplicity.
This thesis mainly focuses on the systematic design and fabrication of the low-cost
micromachined calorimetric flow sensors by using the CMOS-MEMS technology. Primarily, a
unified design platform with the theoretical modeling and simulation was proposed to get a
comprehensive deep study of the thermoresistive micro calorimetric flow (TMCF) sensor
system. Wherein, a general 1D model of TMCF sensor for two types of packaging: the open-space
type and the channel type was proposed. Besides, the equivalent circuit model (ECM) for
the TMCF sensor system was built and validated. This proposed unified design platform could
be effectively applied in the design and optimization of CMOS TMCF sensor system.
Accordingly, two batches of TMCF sensors were successfully designed and fabricated with the
commercial CMOS MEMS technology, respectively.
The batch of monolithically integrated TMCF sensors was fabricated by using a 0.35μm
2P4M CMOS MEMS process. The fabricated TMCF sensors achieved a highest normalized
sensitivity of 230mV/(m/s)/mW with the bidirectional flow detection of nitrogen (-11 ~ 11 m/s),
which was two orders of magnitude higher than the previous CMOS flow sensors. Besides, the
responses of TMCF sensors predicted by the 1D model and CFD model were in good agreement
with the experimental data. Remarkably, an advanced T
2MCF sensor was achieved, where the
common issue of inconsistent output in the thermal flow sensor due to the variation of ambient
temperature T
a was compensated and minimized. The T
2MCF sensor showed the excellent
temperature insensitive output with the maximum normalized variation of 0.5% under the
different T
a of 22°C ~ 48°C. Compared to the uncompensated counterpart (49%), the measuring
accuracy and stability of T
2MCF sensor were greatly improved. Furthermore, a flow regime
map (protrusion d* vs. reduced chip Reynolds number Re*) under different channel aspect ratio
Ar was constructed to serve as a useful guideline for designing a well-packaged, reliable and
accurate TMCF sensor system.
Another wafer-level encapsulated TMCF sensors was designed and fabricated by using the
proprietary InvenSense CMOS MEMS technology. Thereinto, the fabricated Mo TMCF sensor
with the pulsed operated CTD mode achieved an excellent normalized sensitivity of
112.4μV/(m/s)/mW with the bidirectional detection of nitrogen flow (-26 ~ 26 m/s). Besides,
the measured TMCF sensor response time (τ
max < 3.63ms) shows good agreement with the
proposed theoretical thermal RC model. Remarkably, an integrated CMOS MEMS Mo flow
sensor which demonstrates a very compact system on chip (SoC) that features a very low-power
Current Feedback Instrumentation Amplifier (CFIA) as a front-end readout circuit was also
implemented, where the measured sensitivity of this SoC flow sensor was 35mV/sccm. For the
sake of higher-order sensor value, a compact and low-cost micro BTU (μBTU) sensor system
prototype, including a Mo TMCF sensor and a Poly-Si RTD fused on the same chip, integrated
with a digital signal processing (DSP) unit, was proposed. The μBTU system achieved a
detection range of 1.5 ~ 15milli-BTU/h and a sensor system accuracy of less than 6%.
The performance achieved by these CMOS MEMS flow/energy sensors, including the high
sensitivity, low power consumption, large flow range, and rapid response ability, combined
with the high level of integration and low-cost makes them possible to deploy as the
flow/thermal sensing nodes of Internet of Things (IoT) for the flow and energy monitoring in
the smart EeB.
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